I'd create it using multiple layered cloud elements inside each other.  Then use custom procedural shaders on each layer to simulate different light levels, colors and granularity.  This would also have the benefit of allowing you to modify each cloud layer and texture for the animated render.  Sorry, I can't be more specific, but I think you should make custom shaders.  The built ins just aren't designed for this kind of effect.

Quote - Hi all!
So I've been tasked with making a mushroom cloud in Carrara; as in the explosion from a nuclear bomb.
I had a few theories about how to do it, but my ideas aren't working :(
For now I only need it for some still renders, but I have to do multiple views so the could can't be 2D.
My first approach was to model the cloud, but that's as far as I got. I'm not sure how to go about texturing it; I was thinking terrain shaders might do the trick, but I've been messing around for a while and my eye is starting to ache.
Any ideas guys?
Y'all rock!